Process for separating hydrocarbons



, mixture.

Patented Nov. 28, 1944 vuumap STATES PATENT orrlcs mocess FOR serene-ruse muocannons Frederick E. Frey, Bartlesvllle, kla., assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Application 0mm 5, 1942,

Serial No. 460,865

3 Claims. ((1260-6 17) My invention relates to an improved process for separating the components of a hydrocarbon mixture. More particularly, this' invention relates to a process of liquid-liquid extraction in which the hydrocarbon mixture that is to be separated into fractions of unlike composition or properties is contacted with a suitable solvent.

Still more particularly, it relates to a process for the separation of C: to C1 parafilns from closeboiling aliphatic olefins and for the resolution of mixtures oi isomeric Ct to C1 aliphatic h Hydrocarbon mixtures of a complex nature are produced in various petroleum crackin processes and in other ways. In order to utilize to best advantage certain components of such mixtures, it is frequently desirable to isolate orrecover these components in a substantially concentrated form. Thus, for instance, butanebutene cuts that are produced in petroleum refining operations may advantageously bev procselective solvents as *sulfur dioxide, Chlorex requires operating temperatures so low that economically undesirably large amounts of refrigeration are necessary. Thus, as shown in an article by Seyer and Todd publishedin Industrial and Engineering Chemistry," vol, 23, 7

page 325 (1931), sulfur dioxide and various nor mal paraflin hydrocarbons have the following critical solution temperatures:

ar so u on System pep ature I '0. n-Butene-suliur dioxide I -4, 7 n-Hexane-suliur dioxide l0- 2 n-octana-sulfur rlinxirln a 9 11']: Suuur flinxMa I 37. 3 n-Dodccane-suliur dioxide .l 4%.: n-Tetradeeane-suliur dioxida..- n o n-Dotriacontane-suliur dloxide-.-.. 0.

It can be seen that. if buteneswere to be extracted from a mixture with n-butane, using sulfur dioxide as the selective solvent, the opcrating temperature could not exceed -4.'l' C.

Gil

(beta,beta-dichloroethylether) nitrobenzene, an-

iline, dimethyl formamide, and phenol in order to effect separation of relatively less paraflinic hydrocarbons from relatively more paraiilnic hydrocarbons. In general, the solubility relationships of lubricating oil fractions and of the commonly used seleetiw: solvents are such that incomplete miscibility of oil and solvent exists attemperatures that are conveniently employed in a liquid-liquid extraction process. when liquid-liquid extraction of. liquefied inormally gaseous hydrocarbon mixtures or low-boilin normally liquid hydrocarbon mixtures is considered. it is found that in order to maintain the necessary condition of incomplete miscibility of solvent and hydrocarbon mixture it is necessary to work at lower temperatures than in lubricating oil extraction.

In many cases the known method of liquid? mixtures: 55

liquid extraction of light Furthermore, since it is recognized that in general an olefin will have a considerably lower critical solution temperature in a mixture with sulfur dioxide than will the corresponding paraflln, it can be seen that, in liquid-liquid extraction of a butane-butene mixture with sulfur dioxide, an even lower extraction temperature is required, for the extraction temperature must be lower than the critical'solution temperature of the mixture of sulfur dioxide and the particular mixture of hydrocarbons under treatment. A similar situation exists when employing many other known'selective solvents.

It has been proposed to raise the critical solution temperature of the mixture of solvent and hydrocarbon mixture being extracted by adding to the primary solvent a secondary solvent that will impart to the resulting mixed solvent the property of morelimited miscibility or compatibility with the hydrocarbon mixture. However, such a method is open to serious ob- Jections from the standpointof the eflect of a secondary solvent upon both the selectivity and.

in particular, the capacity of a primaryseivent.

Further objections arise from the dimcultiesthat use of -a mixed solvent introduces in the extraction process. such as possible chemical interaction between the solvents, or between sol,- vents and materials of construction, or the need for relatively more elaborate steps .to recover 'vide an improved process for the selective solvent extraction of C3 to C7 aliphatic hydrocarbons at least one of which is an olefin. Another object is to provide such a process for the separation of C3 to C7 aliphatic olefins from closeboiling paraflins. Still another object is to provide an improved process for the resolution of mixtures of isomeric olefins. Still another object is to provide an improved process for the separation of iso C4 to C1 aliphatic oleflns from straight-chain aliphatic oleflns of a corresponding number of carbon atoms. Numerous other objects will more fully hereinafter appear.

I have found that if a liquid mixture comprising light paraffln and olefin hydrocarbons to be extracted with a selective solvent is diluted with a high-boiling oil of low solubility in the solvent, and free from double bonds, the extraction process may be practiced at higher temperatures than would otherwise be possible, and

thus the need for refrigeration may be minimized or eliminated. As a high-boiling diluent may be employed any one of various materials, such as mineral seal oil, paraflinic and/or naphthenic and relatively nonaromatic kerosenes, and the like. As a general rule, I prefer to use as a diluent an essentially paraflinic material of sufliciently high molecular weight to possess quite, low miscibility with the solvent or solvents employed. The diluent may either consist of a substantially pure compound or be composed of a number of different compounds.

The diluent employed according to my process may be a mineral seal oil, oil of the type that is used in absorption processes of making natural gasoline, refined white mineral oil or similar material preferably essentially parafilnic (which includes cycloparafiinic) in nature. The diluent must possess a boiling point considerably higher than the boiling points of the principal components of the hydrocarbon mixture to be extracted so that it is readily separated therefrom by stripping of fractional distillation. Ordinarily its boiling point is so high that it may be readily separated from the solvent in the same manher. In general the diluentis a high-boiling hydrocarbon oil consisting essentially of hydrocarbons ranging upwardly from decane. Its initial boiling point is preferably at least 175 C. and usually 215 C. or higher and the average molecular weight is usually at least 200. While a paraflinic oil of the nature of mineral seal oil or white mineral oil such as "NuloP is pre- .ferred. I may use a somewhat lower boiling essentially paraflinic or naphthenic material such asv kerosene which is substantially free from aromatic components and has an initial boiling point of at least about 175 C. Preferably the diluent is substantially insoluble in the selective salvent and does not form an azeotrope there- W,v th.

Since a considerable difierence in boiling range will exist between the diluent employed and the hydrocarbon mixture to be extracted according to my improved process, only relatively simple fractionations will'be required to separate the diluent from the rafiinate or from the hydrocarbon extract. The diluent so recovered may be recycled and reused for a fresh portion of the hydrocarbon mixture to be extracted.

will

Known procedures may be followed in recovering hydrocarbon extract from the rich solvent. The exact procedure most desirable for any particular case will depend upon the solvent employed and the properties of the hydrocarbons in the. extract. When, for example, sulfur dioxide is used as solvent to extract hexenes from a i traction unit it may be necessary to recover from hexane-hexene mixture to which mineral seal oil has been added as diluent, the rich solvent phase, after withdrawal from the extraction unit, may be fractionally distilled to removesulfurdioxide, which may be recycled to the extraction unit. The resulting. solvent-free residue, which consists of six-carbon hydrocarbons and the high-boilingdiluent, is further fractionally distilled to efiect a separationbetween the six-carbon hydrocarbons" and the high boiling diluent. In certain cases in which the amount of high-boiling diluent dissolved in the rich solvent is negligible, separation of solvent and hydrocarbon extract may be correspondingly simplified. In cases in which a high-boiling solvent such as "Chlorex" is employed, it is preferable to choose a high-boiling diluent of such a boiling range as to avoid including the boiling point of the solvent in the boiling, range of the diluent. However, it is often possible 'to employ a diluent whose boiling range does include that of the solvent by using appropriateknown methods for separating. such mixtures, as for example by steam distillation.

After the rafllnate is withdrawn from the exit dissolved or suspended solvent, as by tree tional distillation or in any other suitable known fashion. Likewise, the high-boiling diluent con-' tained in the raflinate may be recovered by fractionation and returned to the process.

Not only may my improved extraction process be applied to the separation of 'oleflns from paraflins,-but I have found that, in addition, my process may be applied to effect a separation of isomeric straight-chain and branched-chain olefins. Thus, in accordance with my invention, I may effect the separation, into two or more fractions of diflerent compositions; of a hydrocarbon mixture comprisingtwo or more isomeric olefins of different carbon-skeletal structure.

The utility ofthis aspect of my invention is evident, for it provides a way to effect a separation of such isomeric oleflns at moderate temperatures, whereas extremely low temperatures involving excessive refrigeration costs would be required with the commonly used selective solvents according to known processes.

Illustrating the dimculty that would be encountered in carrying outextractionof an olefinic concentrate by known methods is the experimental observation that a liquid mixture of a about 10 per cent butene-l-and about per cent butene-2 is miscible in all proportions with liquid sulfur dioxide at temperatures as low as --74 0. But by the addition of moderate amounts of a high-boiling diluent, accordingto my invention, to such a mixture of butenes, the critical solution temperature may be raised to 0 C. orhigher.

In carrying out my invention, I may employ known procedures and equipment for bringing about contacting of. two immiscible liquids. Thus, one or more stages of countercurrent extraction may be performed in apparatus of the mixer and settler. type, or countercurrent, contacting may be performed in a suitable extracassays:

tion tower. The temperature at which the lieuid-liquid contacting is-carried out will vary, depending upon the composition of the hydrocarbon mixture to be separated, the amount of high-boiling diluent that is added to the nydrocarbon mixture, the particular solvent employed, and the ratio of solvent volume to volume ,toserveasthehish-boilinadiluent. 'Ihecharof the hydrocarbon mixture. In every case, the 7 minimum temperature will be higher than the temperatureof miscibility of the hydrocarbonmlxture and the particular solvent in the absence of added diluent. Also, in every case, the maximum temperature will lie below the critical solution temperatureof the mixture consisting of hydrocarbon mixture being extracted, the

. added diluent, and the particular solvent in use.

The exact value of the critical solution temperature is dependent upon the proportion of high-boiling diluent added to the hydrocarbon ployed, other selective solvents may be used such as phenols such as-phenol (preferably in admix ture with a minor proportion of water), cresol, resorcinol, etc.,' chlorinated ethers such as Chlorexfl aldehydes such as benzaldehyde, rurfural, e'tc., aromatic nitro compounds such'as nitrobenrene, ethylene chlorohydrin (either anhydrous or containing a minor proportion of wam), alcohols such ll mrmm alcohol, ethylene glycol, glycerol, diethylene glycol, anhydrous methyl alcohol, alkyl ethers of ethylene and diethylene glycol, such asmonomethyl ether of ethylene benzyl acetate, methyl iuroate, wrcinol monoacetate, 'etc., N,N-dialkyl amides suchas dimethyl formamide, etc.

Although my improved extraction process is applicable to the separation of a wideranse o! paraiiin-olenn mixtures, or of mixtures of isoacterlstics'oi this oil were about as follows:

Gravity, "A. P. I. 27.0 Color (Saybolt) 20+ Pour point -..-F.. 0 Saybolt viscosity (100 1'.) sec-.. 106 Flash poin TL. 325 Fire poi TL. 305 solub ityin liquid sulfur dioxide (25 6.)

...volume per cent..- 1

The composition of the hydrocarbon-sulfur dioxide mixture after addition oi the light oil as diluent was as iollmmin per cent by weight:

Butane-batches -4- 31.5 light oi 16.4 Sulfur dioxide 52.1

Byexperimentitwasfmmdthatthismixtum' formed two'liquid phases at temperatures below about 5." CI, and became homogeneous at higher temperatures. The actual extraction was periormed by cooling to +10 C. and asitatins for a sumcient time to ensure equilibrium, after which the phases were separated; the light oil s was separated by distilling each phase; and

meric oleflns oidiflerent carbon-skeletal 'structure, it is particularly pplicable to the separation oi such mixtures containing hydrocarbons havin; from three to seven carbon atomsfper molecule, and it possesses its greatest usefulness in separatins --My invention may be further illustrated and described by thefiollowhig speciilc cxamples:

Imus:

close-boilins mixtures of suchhyd'ro- 7 j A we bstchwisc liquid-liqmd extractionwas carried out on a refinery butane cut or the tollowing composition, in moiepsr cent:

i-Butene 23 n-Bu'tenes 85.7 Butanes 41.3

Liquid suliur dioxid'ewasused asthesolvsntin theextractimlnpreliminarycxperimenisit was found that the critical solution temperature oi the butane-butsne mixture'with liquid sulfur dioxide was about --85 C. In order to raise the miscibility temperatm-e of the system, there was analyses then were made on theresulting distillatemixtures of sulfur dioxide and butane and butenes. The'compositions of solvent and hydrocarbon phases were found to be as follows:

It is seen from that a oon'siderabl enhancement in the olefin content of the butanebutene extract the mists contacting; whereas the butsncs in original butanebntene mixture to 41.3 mole percent,

the butanes in the extrsctamountodto only 35.5 mole per cent. 'Ihus'by the additions! the light oil as diiuenait was possible to conduct theextractionsucoessmllyatatempersture 25' C. abovethecriticalsohition temperatureof the sulfur dioxith and the bntane-butene mixture. Ann-ther-concentrationoioleilnsinthe extract could havsbeen obtained by successive stages of extraction. 1

The foregoing extraction data also illustrate the phase of. my invention which deals with the separation of isomeric straight-chain and branched-chain olenns. It is seen that the mole ratio or isobutenc to normal butenes-in, the orla'inal butane-butane mixture was 0.644, whereas the ratio in the extract was 0.724; thus. apartial separation or the isobutcne from the normal butanes was effected, and separation could have been made more'neariy complete by reposting the extraction. a number oitimcs. By I suitable choice oi conditions, depeudingupon sol-' added a measured amount of a light neutral oil Vent! and chs se' i it 18 M according Holcpcr g? 111L111 1 e2 .2. 10:7 3114 10. 1 can map no.0

lib-0 H to my invention, either to separate olefins from paraflins or to separate isomericbranched-chain olefins from straight-chain olefins.

Example If A single batch-wise liquid-liquid extraction with sulfur dioxide was made on a second portion of the refinery butane cut described in Example I. Before performing the extraction, there was added to the refinery butane cut a quantity of light neutral oil as diluent; this light oil was of the same composition as that described in Example I. The composition of the total sulfur butene mixture. A more complete separation would be obtained by further stages of extracdioxide-hydrocarbon mixture, when brought to-' I gether for extraction, was as follows, in per cent by weight.

" Butane-butenes 22".?-

Light 011' p 37.1 Sulfur dioxide 40.2

By preliminary experiments it was determined that. the miscibility temperature of this mixture was about 9.8 C. and that two liquid phases existed at lower temperatures. The actual extraction was carried out .at 0C. by a procedure like that described in Example I.

countercurrent operadicated in the following c After removal of the light oil diluent the compositions of the hydrocarbon and the solvent phases resulting from the extraction were found tobe as follows:

. Mole rcent Mole percent on S rfree bmis Solvent phase:

Sulfur dioxide 79. 2 0 i-Butene 6. 2 29. 8 n-Butenes i- 8. 3 39. 7 Butanes 6. 3 30. 6

Hydrocarbon phase:

- Sulfur dioxide 46. 3 0 i-Butene l1. 9 22. 2 n-Butenes l8. 8 35. 0 Butanes 23. 0 42. 8

It is seen from these data that a selective separation of butencs from butanes resulted from the extraction, and this result was obtained at a temperature about 35 C. above the critical soluttion temperature of the solvent and the butane- I claim:

1. A process of effecting the separation of isomeric close-boiling aliphatic C4 to C1 olefins of different carbon-skeletal structure from one another which' comprises diluting a mixture of said isomeric aliphatic olefins with an essentially parafllnic high-boiling oil free from double bonds and-having an initial boiling point of at least 175 C. in an amount such that the critical solution temperature of the mixture of the resulting mixture, and sulfur dioxide described below is substantially above the critical solution tempera-.

ture of the same mixture without said oil, ex-

- tracting the resulting mixture with sulfur dioxide at a temperature below the critical solution temperature of the diluted olefin mixture .and the sulfur dioxide but substantially above the critical solution temperature of the olefln mixture and the sulfur dioxide in the absence of said oil and at a pressure suflicient to-maintain liquid phase conditions and thereby effecting preferential dissolution of the more branched olefin content of said mixture in the sulfur dioxide phase and concentration of the less branched olefin content of said mixture in the hydrocarbon phase, separating said phases, and

'recovering'the olefins from the separated phases.

2. The process of claim 1 wherein said more branched olefin content is isobutene and said less branched olefin content is normal butene. 3. The process of claim 1 wherein said more branched olefin content is isobutene and said less branched olefin content is normal butene and wherein said extraction is carried out at 0 C.

FREDERICK E. FREY. 

